Initial conditions - airspeed, Mach, Reynolds, airdensity etc.

[Alan Teeder]

For my VC10 model I want to run VSPaero at different flight conditions.

On the Overview screen Mach Start (and Mach End  for a sweep) can be set.

On the Advanced screen,  Vinf, Rho, ReCref, and if the box is ticked, Vref and Mach Ref can be set.

In the real world these are all interrelated.

I assume that VSDPAero has no inbuilt Standard Atmosphere, so for a given  Mach No. and altitude,  I need to use my standard atmosphere tables to calculate airspeed,  density and Reynolds No.  I also assume that, with my model using Imperial measurements,  Vinf is true airspeed (ft/sec) and Rho is air density (slugs/ft-3). Are these assumptions correct?

What is the intended use of Vref and Mach Ref.?

2nd order Karman-Tsien Mach correction can also be ticked. Are there any guidelines on the use and validity of this?

Thanks for your support.

Alan

[Rob McDonald]

Don't think of velocity, density, Mach, and Reynolds as things that are connected in the way they really are.

In a potential flow code, things are a bit different.

Traditional simple potential flow codes model inviscid and incompressible flow.  Pressure, force, and moment data is all output in coefficient form.  Because of all this, the velocity does not matter.  It is often taken to be 1.0 -- and often is not even a user controlled input.

When you add a compressibility correction to the potential flow equations, you introduce the Mach number -- but it does not have the same role as the velocity.  You linearize the equations about the freestream Mach - and that is it.  You can set Mach to 0.5 and still have velocity not matter.  Think of Mach as a knob that controls the influence of compressibility.

Next, when you add some sort of viscous model, you introduce the Reynolds number -- but it is not really linked to viscosity, density, velocity, etc.  It can be set separately and it then goes into just the places where viscous effects are important.

So, each of these terms is introduced to help capture some additional physics -- they don't have to play their fully coupled role.

Velocity in VSPAERO is mostly there for unsteady flow.  Whether your model has rotating blades or you are doing an unsteady roll/pitch/yaw analysis, you need the forward velocity to relate the unsteady motion to the steady motion in the units of the model.

VSPAERO has Reference quantities for Velocity and Mach to allow static / hover analysis of rotating blades.  In those cases, the Velocity and Mach is zero, but you still want to compute coefficients (V^2 in the denominator) that don't blow up.  So, you will probably pick the rotational tip speed and Mach number as Vref and Mref.

Density is the most tricky one in VSPAERO.  I believe it is only used in the actuator disk calculation as it is needed to convert the thrust and power coefficients input into dimensional forces and to calculate the velocity field induced by the actuator disk.

VSPAERO will work with any units - so long as you are consistent.  This is easiest with ft, lbf, slug, s.  Pretty easy with m, N, kg, s.  More complex for inch or cm or anything else.

Rob

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[Alan Teeder]

Thanks for the reply. I´m glad you kept it nice and simple ;)

I will do some simple runs with different airspeed/density/Re/Mach etc. settings to see if the results are in line with my understanding. I can cross-check these against Datcom and see if they show similar trends.

Alan

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[deenri...@gmail.com]

Hi, 

 does the Reynolds number correction simply apply in the CDo term? As in, some skin friction effects are added based on the input Re?

Thanks for the help!

- Daniel

[Rob McDonald]

Yes, it is only used by the skin friction terms in CD0 (and the skin friction terms in rotor power for rotating blades).

Rob

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