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42 votes
Accepted

How exactly did engineers come to the final design of jets like the F-16 or SR-71?

The short answer is that if we can program it into a computer then we can do it manually, it just takes a lot longer. Back in the day, a "computer" was a clerical professional or ...
Guy Inchbald's user avatar
  • 7,062
33 votes

How exactly did engineers come to the final design of jets like the F-16 or SR-71?

The A-12 and SR-71 were flat like an ironing board for a reason. This way, the pitching moment at Mach 3+ was easy to calculate. At the time of the design, Mach 3 wind tunnels were rudimentary and had ...
Peter Kämpf's user avatar
24 votes
Accepted

Will computational fluid dynamics (CFD) ever be 100% correct?

This is a broad question, hence a broad answer 😃 As high eng CFD now stands, it already is correct enough for, say, 95% of use cases. I am not aware of much utility in modelling complex turbulent ...
Jpe61's user avatar
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11 votes

Will computational fluid dynamics (CFD) ever be 100% correct?

100% correct, no. That's because CFD (and a great many other computer applications) work on numerical approximations. There's either no exact solution* known, or the exact solution isn't computable. ...
jamesqf's user avatar
  • 2,577
10 votes

Is turbulence a random process?

The equations that describe fluid flow, the Navier Stokes equations, are generally considered chaotic. In particular turbulence is defined as " fluid motion characterized by chaotic changes in ...
DJClayworth's user avatar
  • 4,501
9 votes

Will computational fluid dynamics (CFD) ever be 100% correct?

First, the Millennium Challenge asks about the existence and uniqueness of a solution to the Navier-Stokes (NS) equations given any compatible initial/boundary conditions. This question is of ...
JZYL's user avatar
  • 11.1k
8 votes
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How high could modern transonic commercial transport designers reasonably push L/D ratios?

First of all, I am sceptical regarding your graph. The Fokker F-28 or the Short 360 are way too good while the Boeing 767 looks poorer than what other sources claim. Size alone helps already by making ...
Peter Kämpf's user avatar
7 votes
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How has Computational Fluid Dynamics improved commercial airliner wing design?

I'm unsure as to what specific wing characteristics changed as a result of the application of CFD in the process of wing design. Look at the change in airfoils over span of a swept wing. A simple ...
Peter Kämpf's user avatar
7 votes

what speed was the "negative 4G inverted dive" in Top Gun

The maneuver described is so non-sensical that I don't think knowing an airspeed range would help. In fact I don't think you could properly model it at all... Start by asking yourself what the words &...
Michael Hall's user avatar
  • 26.9k
4 votes

If Navier-Stokes equations are unsolvable how does CFD software calculate simulation?

The Navier Stokes cannot be solved analytically, they can however be computed numerically. Which is the technology behind flight simulation, using a determined time interval to compute all dynamic (...
Koyovis's user avatar
  • 61.8k
4 votes

Will computational fluid dynamics (CFD) ever be 100% correct?

No, it will never be 100% accurate. Air or water are quite well described by the Navier-Stokes equations assuming the fluid is a continuum. Usually it is also assumed that they are Newtonian fluid. ...
Vladimir F Героям слава's user avatar
4 votes

How is CFD used in aircraft design? Qualitative vs Quantitative?

In the industry CFD is used for both of the purposes you describe. CFD simulations are run on existing and proposed designs in order to understand the flow over them, and then based on that ...
DJClayworth's user avatar
  • 4,501
4 votes
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Does Navier-Stokes "recognize" existence of induced drag, can we get induced drag from them?

NS CFD will 'Recognize' induced drag in the sense that it can calculate it accurately (when done right). However, NS CFD will not separate induced drag out from other sources of drag -- including ...
Rob McDonald's user avatar
  • 13.5k
4 votes

How do I know which outer grid shape is best for CFD?

These grid topologies refer to structured meshes. Unstructured meshes are a different technology with different pros/cons, I won't discuss them here. A mesh must have adequate resolution to resolve ...
Rob McDonald's user avatar
  • 13.5k
3 votes

How the placement of the pitot and static ports decided?

Both the pitot and static pressures will vary a lot based on: Airspeed CG location Angle of bank Angle of attack Configuration, especially flaps In the CFD model, and then again in flight test, you ...
Max R's user avatar
  • 3,808
3 votes

How is CFD used in aircraft design? Qualitative vs Quantitative?

The quick answer is it depends. As for the small businesses doing aircraft design generally sweeps or trades will be run on a low to mid fidelity multidisciplinary design code to map the design space. ...
spacegirl1923's user avatar
3 votes
Accepted

How is CFD used in aircraft design? Qualitative vs Quantitative?

It really depends on what your objectives are. For design optimization, the currently available automated optimization methods (Adjoint and other topology methods) are not as good as a well ...
Stuart Buckingham's user avatar
3 votes
Accepted

If Navier-Stokes equations are unsolvable how does CFD software calculate simulation?

The equations solved for the most of the CFD software are the Navier-Stokes equations (let's stay in the general case of compressible ones). The Problem of Navier Stokes equations solution existence ...
albiremo's user avatar
  • 491
3 votes

Will computational fluid dynamics (CFD) ever be 100% correct?

For the reasons cited above, the answer definitely No, but one point to remember here is that high-end CFD does not have to be perfect to be immensely useful. Imagine one wanted to get 7-place ...
niels nielsen's user avatar
3 votes
Accepted

What determines CFD accuracy for aircraft design?

CFD is a prime example for "garbage in – garbage out". There is a large range of parameters that needs to be set correctly, from fluid properties to surface roughness. If the persons feeding ...
Peter Kämpf's user avatar
3 votes

Is the math behind my approach to design a propeller right?

You're using a uniform thrust condition to set chord. The Betz condition for a minimum induced loss propeller (best efficiency) is a uniform induced velocity. You may want to look at reformulating ...
Rob McDonald's user avatar
  • 13.5k
2 votes
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Why aren't thermal analysis of propeller done?

As a propeller turns through the air at subsonic speeds, the dynamics of creating the pressure distributions in the air surrounding the propeller blade can be accurately modeled as being isothermal (...
niels nielsen's user avatar
2 votes
Accepted

How do engineers estimate the aerodynamic effects caused by jet engines or propellers in wind tunnels?

Turbofan and propeller thrust is known from the manufacturer and does not normally need to be included in wind tunnel testing. Models usually represent solid structure, so they have: hollow nacelles ...
Pilothead's user avatar
  • 20.7k
2 votes

How to input Reynolds number and Mach number as input boundary conditions in Ansys Fluent

Disclaimer: I am not familiar with ANSYS, so this is just plain physics. You need three parameters to describe flow conditions: air speed air density at rest air temperature at rest then all other ...
Peter Kämpf's user avatar
2 votes

How has Computational Fluid Dynamics improved commercial airliner wing design?

For the last twenty five years, at least, every significant development in aerodynamics has involved computational fluid dynamics. Nobody has designed an aircraft, a wing, or even a car in the last ...
DJClayworth's user avatar
  • 4,501
2 votes
Accepted

How do I get rid of this chord line along the x-axis when importing an airfoil into XFLR5?

You most likely are importing the points with the wrong order. Usually programs are looking for points to start at the TE and go clockwise along the lower surface to the LE and then back to the TE ...
Rob McDonald's user avatar
  • 13.5k
2 votes

Simulating a tip vortex

I have read on this forum that the tip vortex comes from the roll-up of the vortex sheet behind the entire wing as opposed to being a local tip phenomena You've read good 👍 This picture (Wikipedia) ...
sophit's user avatar
  • 13.8k
2 votes
Accepted

Flow separation in inviscid flow

Euler CFD solutions are subject to something called artificial (or numerical) viscosity. How much you have depends on whether your method is odd or even order -- dictating whether a diffusive or ...
Rob McDonald's user avatar
  • 13.5k
2 votes

Why would there be a drag discrepency when simulating transonic flow over scaled CRM geometry?

First, have you done a grid resolution study? Not just the boundary layer, but everything? Shoot for a halving and doubling of your mesh (i.e. one coarse and one fine) and then try to apply ...
Rob McDonald's user avatar
  • 13.5k
1 vote

Interpreting a graph depicting airfoil technology improvement

I don't think this graph is supposed to be very quantitative, otherwise there would be a "real" scale on both axes instead of the two-ended arrows and the hatched lines would be actual ...
Raketenolli's user avatar
  • 1,047

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