SolidWorks UK

How long is the flow?

I was asked yesterday if it was possible to calculate how long the flow lasts inside the domain during a flow simulation? Think of this another way and it is easy to solve; how long does it take a particle to pass through the domain when it enters the flow inlet.

To do this, we therefore start off by performing a particle study:

1. Insert a new Particle Study
2. Define an injection of particles from the inlet
3. Run the Particle Study

Then, to find the time of a particle in the domain, simply RMB on the Particle Study created and View Results.

Hit the Summary button,

and you will get a table of results for each particle injected into the inlet.

 

The thing you are interested in here is the Residence Time, which is the “life time” of the particle, or how long it exists in the calculated domain.

However there are some other interesting details here too.

The Length is the physical distance travelled by the particle whilst in the domain. You will notice that each particle has a different length and residence time, and this is simply due to the fact that each particle will take different paths depending on their starting position in the flowstream.

The picture below shows this clearly as some of the inlet particles travel all around the circumference of the external pipe, whereas others travel more directly from inlet to outlet.

This means that you should consider if you need to know the min, max, or average of the residence times for your particular application.

You will also notice that the Summary table gives you the Fate of each particle and this leads us to another interesting option in Particle Studies.

If you go to the Boundary Conditions tab in the Particle Study setup and edit the default setup, you will see three options for the boundary conditions.

This determines what happens when the particles meet the walls of the domain, and the options are:

• Absorption - The particles are absorbed by the walls. This is typical for liquid particles.

• Ideal reflection - The particles are reflected from the walls. This is typical for solid particles.

• Reflection - Specify the normal and tangential restitution coefficients which are ratios of the absolute value of the normal and tangential velocity components correspondingly after and before the collision:

So the Fate of the individual particles may be to reach the opening, be absorbed, or possibly to end their time still within the domain when the calculation finishes.

Technorati Tags: SolidWorks Flow Simulation,Tips and Tricks,Particle Study,Residence Time

Welded to the spot – and now edge…

One of the most common questions from people who have welded structures is, “can you model welds?”. Well, obviously!

In 2010 we have a new edge weld connector, similar to the spot weld connector already available.

The tool allows you define a fillet or groove weld, either single or double sided between any two sheet metal parts or shell structures like the one below.

By picking two intersecting surfaces the edge weld is generated for you at the intersecting curve.

Now all you have to do is pick an electrode material from the list of steel and aluminium types, and provide an estimated weld size.

The weld is then ready to go!

Simply solve your simulation as usual and the forces and moments are automatically passed through the new weld onto the surrounding structure.

However the really neat bit comes next. Not only is the weld connector a new joint type, it currently compares the forces and moments through the weld to the AWS 1.1 standard and tells you if the weld size you have chosen is good enough!

To get this info, RMB the Results folder and chose Define Weld Check Plot. Hit the OK tick and you get a new dialog that lists all your welds into two groups, those that pass and those that fail according to the standard.

This allows you to browse each connector and see where you need to define a larger weld.

What is more, if you chose a weld in this dialog and hit the Details button, you get even more helpful info containing the required weld and throat size plus the forces and moments experienced by the weld in the particular configuration you tested.

And as a final benefit, you can also plot the required weld size and throat size along the weld length (from an indicated start point and direction) based upon the loads generated from the simulation.

 So you can see that with a few short steps it is possible to assess your design for weld sizes. In fact you could put any arbitrary weld size in, because the software then tells you what is ultimately needed, which is of course what you want to know!

Technorati Tags: SolidWorks Simulation,Simulation,Welds,Simulating Welds

The Future of SolidWorks Simulation?

It’s been a while since I’ve posted a simulation blog as I’ve been busy looking after my new daughter. As proof, here’s a picture showing just what a fan of SolidWorks she’s already become :-).

So enough about me, let’s get back into the swing of things with a couple of insights this week.

Firstly I received this fantastic video of a flow simulation showing a timing sequence of valves in an engine manifold.

I particularly like the simplicity with which the pressures can be switched on and off using the dependency button. Very impressive.

Up next, as part of our SolidWorks 2010 launch we have been showing some of the great new motion simulation features.

One of the main reasons for using motion simulation is to know the loads that your mechanism experiences when its moving. After all, as the mechanism moves, the loads will change at each moment in time putting varying strains on your design. How do you know if you have tested the worst case?

In 2010 we have a new tool, called stress recovery (because that is what it does). Notice the three new buttons on the motion dashboard…

The first button on the left provides a dialog allowing you to choose which components you want to see stressed, and of course which period of the motion you are interested in (perhaps the entire motion just in case you can’t decide when the worst case is!).

Having defined this and run a motion study as normal to generate the loads, the middle button    then launches the structural solver to calculate the displacements and stresses on your chosen part at each time step.

And the final button on the right let’s you pick which contour you want plotted.

So, with hardly any effort, and moving from right to left through these new buttons, it is simple to generate a proper understanding of the response of your mechanism to the changing loads generated in a motion study.

 

Technorati Tags: SolidWorks Simulation,Motion Simulation

 

Are you up to the challenge?

Challenge your knowledge on 6 common design problems:

 
Good luck and enjoy the quiz.

Its not just about Simulation you know!

Very often when encountering problems with a simulation, it is all too easy to look at the simulation inputs for a solution. For example, if the model doesn’t mesh or solve you immediately think, “is my mesh too coarse?”, “are all the parts bonded correctly?”, or even “am I using the right solver?”.

However one very important variable in the success of a simulation is the CAD model itself, which becomes very sensitive if it hasn’t been built with analysis in mind. A common example of this is when someone gives you a model converted from another CAD source or from someone who has no idea that simulation will be done on their assembly.

I came across such an example last week when working on a file that had been converted from another CAD package. The model seemed to mesh ok but when I came to solve it I got an error that read:

Now the solution to this problem is usually to do with parts not being properly restrained (as it says), leaving rigid body motion a distinct possibility.

A common next step is to run the same model through a frequency analysis, because this will happily solve for the rigid body modes and help identify where the failed connections exist. However in my model the Frequency Study wouldn’t solve either giving a similar error!

Having checked and re-checked my simulation setup for the correct boundary conditions I ended up back at the geometry, which is where the problem was finally discovered. Because various CAD software uses different tolerances during CAD creation, an imported file from another source may struggle to define global bonded contacts within SolidWorks.

To check the contacts, use Tools > Interference Detection, pick the assembly and hit the Calculate button. A list of all inter-part contacts should be given that allows you to select each one and check out the contacting areas of your model.

It is useful to note that if you have the ‘Treat coincidence as interference’ ticked and you get a series of interferences, but switching this off reveals no interferences, then the global bonded contact definition should work for you. This was the case I had in my solid model, yet I was still getting a failed solution! However going through this interference list step by step eventually highlighted the problem.

About three different regions displayed a single straight line, rather than an area as the interference region between two parts. This is shown as a thick red line (below), which is much more obvious than the other shaded areas of interference. And no matter how closely you zoom in to this line, it never enlarges to the typical shaded area.

Clearly these thick lines indicate areas of the geometry where bonding is applied

along an edge. This in turn implies a zero area for the stress to be calculated on and so an infinite result would ensue, something we can’t have.

To fix this problem I chose to create a new merged part containing all the separate components in the assembly so that the other touching faces would transmit the load and no bonded contacts would need to be used.

The moral of this story is that doing simulation needs a solid basis to work from. Don’t forget to check the integrity of your CAD model, especially if it was originally built with no concern for simulation.

Technorati Tags: SolidWorks Simulation,Tips & Tricks,Solver Errors

SolidWorks Simulation 2010

I’ve just spent the past couple of weeks with our Northern European resellers going through some of the new simulation features in 2010. Feedback has been amazing and there are some really neat technologies that I’m sure we’ll be talking about soon.

However we have also spent quite a bit of effort making the day to day tasks of using the products more simple, intuitive, and productive and so I wanted to begin by highlighting one of these.

Firstly think beam modelling, think pre-processing, and think what a pain it is when you have to select specific surfaces of a beam geometry to set up bonded contacts.

In the example above the two side members of the bridge structure support the footplate, which is modelled as one complete curved surface (highlighted below).

Therefore we need to create a bonded contact set between these three entities. In SolidWorks 2009 this would have required the user to pick the top face of each side beam to bond with the surface of the footplate, meaning a repetitive ‘Select Other’ command to drill down to the correct beam face.

This might be ok for the two side members,  but what if you had to do this for each cross member too?!

Well, in 2010 the whole process has been made more intuitive and more intelligent by allowing the user to simply pick the entire beam rather than having to select the correct face of the beam.

Here we see the bonded contact set definition requiring the two side members and the surface of the footplate alone. No mention of the beam surfaces and no need to have to drill down through layers of geometry to find them. The software automatically checks for proximity of these items and applies the bond to the correct face.

Quick, clever, and productive! Now that I like.

Technorati Tags: SolidWorks Simulation,Simulation Tips & Tricks,Beam modelling

Confusing terminology

Sometimes you come across a term in simulation that doesn’t make much sense and seems to have been put there to confuse and confound. One such term that I was asked about recently is the purpose of the Jacobian check, a mesh checking tool that I know hardly anyone uses but is one of those things that can be very useful.

The whole purpose of checking your mesh is to see if it is well defined. A highly deformed element becomes inaccurate and a mesh created with whole regions of badly defined elements will just serve to lead you astray. And where do elements tend to become deformed? In areas of highly curved geometry, which is often where you are interested in knowing the results.

So what is the Jacobian check I hear you say? Well, consider a normal tetrahedral element with mid-sided nodes.

If those mid-sided nodes lie directly between the corner nodes, then the element is not deformed. However, the moment you begin to move the mid-sided nodes away from this line, to track the edge of a piece of geometry say, then the element begins to deform.

The Jacobian check is a measure of how deformed your element has become. A perfectly straight sided element has a Jacobian of 1.0 and is ideal, and the Jacobian ratio increases as the curvature of the edges increase.

The actual calculation is done based on a series of points inside the element called Gaussian points. The user can choose how many of these points to use in the Jacobian check when they create the mesh by selecting 4, 16, 29, or the node locations as the points at which to base the calculations on.

So performing a mesh quality check for the Jacobian generates a contour plot allowing you to identify areas where you may need to increase your mesh density, replacing a few badly deformed elements with a greater number of better shaped elements.

In the example below my test component has firstly been meshed with a course mesh and then a Jacobian check has been performed. The legend reports a Jacobian ratio of 1.0 as blue and shows how the main body of the straight edged geometry can be nicely modelled with straight edged elements. But the hole in the middle requires the element edges to become curved and so the Jacobian ratio increases, here to a value of 1.59.

Simply remeshing with a fine mesh improves the maximum Jacobian ratio to 1.19 showing how more elements around the circle mean they are less mishapen.

It is generally accepted that a Jacobian ratio below about 40 is acceptable in most cases so you have to balance mesh density with run-times, perhaps refining the mesh with controls in certain areas rather than across the entire part. And as I’ve said before, always have a sanity check and ask yourself if the highest Jacobian ratio is in an area where you will see the highest stresses anyway.

Technorati Tags: Tips & Tricks,SolidWorks Simulation,Jacobian Ratio

Getting dropped

Preparing to demonstrate a drop test the other day, I was reminded of a useful procedure for creating a suitable mesh and thought you might like to know about it. 

1. Identify the surface that will hit the ground first during the drop test.
2. Create a static study.
3. Add a fixed restraint to the surface identified in the first step above.
4. Apply a reasonable gravity loading (e.g. 10g).
5. Then run an h-adaptive study with a fine accuracy setting.
6. Create the drop test study and then drag and drop the mesh from the static study on to it.
7. Continue with the drop test.

By using an h-adaptive mesh, you should find the elements are refined in the regions most suitable for the drop test. Sneaky but cool.

 

Technorati Tags: SolidWorks Simulation,Drop Test,Simulation Tips & Tricks,Meshing

How to calculate the flow coefficient of a valve

In the past couple of months I have seen two examples of customers interested in the calculation of the flow coefficient of a valve. Known as Cv, the flow coefficient is a relative value that allows us to compare the efficiency of different valves regardless of their size or type.

Before we see how to simulate this using SolidWorks Flow Simulation it is important to note that the calculation of Cv differs for gases, liquids, and even steam and can change depending on the pressure drop across the valve. For a list of the relevant equations look up http://www.engineeringtoolbox.com/flow-coefficients-d_277.html.

However in essence the equations follow the same basic principle,

                        

where q is the flow rate, SG is the specific gravity of the fluid, and dp is the pressure drop across the valve.

So to model this in Flow Simulation, simply define the pressure drop across the valve with explicit inlet and outlet pressure conditions and measure the flow rate.

For example, you might apply an environmental pressure of 16 bar at the inlet, a static pressure of 15 bar at the outlet, and measure the flow rate at the outlet.

Also, by defining the inlet and outlet pressures and flow rate as goals, it is pretty easy to set up an equation goal that will automatically calculate the Cv value for you.

Here is the equation goal I used for air flowing through a valve with a non-critical pressure drop. Whilst it looks daunting, it is actually just referring to a whole load of other goals picked directly from the list in the flow simulation analysis tree so it’s not that difficult.

I want to finish by mentioning a couple of points. You have to be very careful to use consistent units when calculating the flow coefficient, and the Imperial system is common. Also keep an eye on definitions, for example, the term for q that I used is actually required in ‘standard cubic feet per minute’, which is not the basic cfm calculated by flow simulation. Here I have factored it to standardised conditions of temperature and pressure in the first few terms.

q = ({Average Outlet Pressure, Pa}/101325)*(273.15/{Average Temperature of Fluid, K})*{Outlet Volume Flow Rate, cfm}

And finally it is good practice to calculate the Cv value for two or three different pressure drops and take the average, just as you would with a physical test.

Technorati Tags: SolidWorks Simulation,Flow Simulation tips,Cv value,Valve flow coefficient

A good reaction?

When performing a motion study, there are obviously a number of moving parts all connected with each other, pushing and pulling in response to the driving forces set up.

Reaction forces and moments are frequently plotted at joints but it can often be difficult to work out if they make sense when only presented with a graph.

 

A more visual clue to understanding what is going on is the Reaction Force Vector.

When you create a reaction force plot as normal, you are asked to pick the joint in question (say Concentric2 below). However whilst you are in the same selection window, if you also pick the face of one of the connected parts a new option appears (as if by magic) at the bottom.

The option to ‘Show vector in the graphics window’ appears and picking this displays the vector for the reaction force or moment.

      

This vector can be used to get an appreciation of how the forces act in your model and updates dynamically when the model is re-run or re-played. Quite useful when trying to understand how all the joints carry the load in a model, and another sanity check when you Import Motion Loads to a stress study.

Technorati Tags: Simulation tips & tricks,Reaction forces,motion study