Why is flow unsymmetric over a delta wing?

1. Jan 13, 2016

Algren

I understand that there are vortices formed on the delta wing for moderate angles and low speeds. I observed recently 2 asymmetric vortices forming on a small delta wing model for a moderate angle and a low speed. But, why would there be asymmetriic flow over a delta wing? If the flow is approaching the wing symmetrically and the wing itself is symmetric about the same plane, the flow should be symmetric, right?

And ofcourse, i'm talking about the plane at right angles to the wing and containing the root chord in it.

2. Jan 13, 2016

MrAnchovy

But it isn't due to the angle of attack.
But it isn't - even if the camber is symmetric (which it isn't usually) the control surfaces create asymmetry.

3. Jan 13, 2016

Algren

The case i'm talking about, there are no control surfaces, just a triangular slender sharp edged delta wing. The model which i tested in is also very small and simple, and its basically a triangular prism which tapers at the leading edge.

WHen it comes to angle of attack, i'm talking about symmetry ACROSS the plane PERPENDICULAR to the wing and CROSSING the root chord.
You are talking about symmetry across the plane in which the delta wing itself lies, and i'm not talking about that plane.

4. Jan 14, 2016

MrAnchovy

Which?

If you are saying that the vortices off each tip of the wing pair are not symmetrical then this may be down to swirl in the wind tunnel flow: how sophisticated is the wind tunnel?

5. Jan 14, 2016

Yeah my first impression here is that this is experimental error either due to the measurement technique, the mode, or the tunnel. For a delta wing that is, in fact, symmetric at zero yaw in a uniform free stream, the two sides should be symmetric.

6. Jan 14, 2016

Algren

Alright, thank you for your clarifications. So i guess either the wind tunnel had some asymettricities in it or our delta wing might've been placed at some yaw. My doubts have been cleared.

7. Jan 14, 2016

What sort of wind tunnel are you using? Is it homemade? Is it one used for research at a university? What is its configuration?

8. Jan 14, 2016

David Lewis

If the model is light enough, and the CG is in the right place, you can test glide it to discover flaws in the model and trim out asymmetries. At very low Reynolds number, the delta wing's boundary layer is chaotic, unpredictable and sensitive to minor deviations in surface contour or texture.

9. Jan 16, 2016

Algren

Its one of the smallest wind tunnels in the institute, open suction and max 15 m/s. it's got a small test section cross section, like 1 ft squared or something, and its got a 9 contraction ratio.

So, my professor said that its because the navier sokes equation isn't symmetric for high Re flows, now im confused again :(

10. Jan 19, 2016

Well for one, given the tunnel parameters, this is nowhere near a situation that could be considered "high Re." At room temperature the unit are is only in the ballpark of $10^6\;\mathrm{m}^{-1}$. Even a model that's a long as your test section is wide would only have $Re_c \approx 3\times 10^5$, which is quite small. It almost certainly wouldn't even be turbulent.

Second, there's nothing about the Navier-Stokes equations that says they would become asymmetric in a symmetric domain. It honestly just sounds like your professor doesn't know the answer and is making up reasons.

What are the dimensions of your model? Is it mounted in the center of your tunnel or off-center?

Last edited: Jan 19, 2016
11. Jan 19, 2016

cjl

I agree with boneh3ad here - there is nothing about the Navier-Stokes equations that would indicate a symmetric model at zero yaw or roll would have any asymmetries. It is likely a problem with the experimental setup - what's your model size, and how is it mounted in the tunnel?

12. Jan 19, 2016

FactChecker

It's very possible that the right and left flows can not coexist symmetrically and one will dominate or be different from the other. They may alternate in their behavior. The situation of symmetry would require that the flow is statically stable. It is similar to the situation when a flag is being blown on. It doesn't go straight back. It flaps.

13. Jan 19, 2016

The flag example isn't really relevant to your point. In fact, the flapping of a flag (or any similar phenomenon such as von Kármán vortex shedding) may not appear symmetric when time-resolved, but they actually are statistically symmetric. The flow structures on each side will develop identically and at the same rate; they will just alternate back and forth. The net result is statistical symmetry.

In other words, even if there was some shedding/alternating type phenomenon occurring, it would almost certainly still be statistically symmetric. Of course, it's highly unlikely that is happening anyway. There is nothing on the OPs geometry that should give rise to that.

The more likely culprits are things like non-uniformity in the free stream flow, a blockage effect due to the model being too large, upstream effects of the mounting system, or other such systemic errors.

14. Jan 20, 2016

FactChecker

Are you saying that the asymmetric behavior must be cyclic and "statistically symmetric"? I don't see why.

15. Jan 20, 2016

Because the domain is symmetric, and therefore so are the forces on the flow. If it overshoots one way it will tend to move its way back to symmetry again. Just look at the vortex shedding behind a cylinder. This is all assuming that the domain and incoming flow are symmetric. If not then there's no reason to expect the flow to be symmetric.

16. Jan 20, 2016

Algren

The test section dimensions:
16x12x39 inches, 39 is the length of section.
The dimensions of delta wing:
Root Chord: 220 mm, Span 160 mm..
At a velocity of 4 m/s and AOA of 27 degrees, we observed one vortex bursting a bit upstream of mid chord and the other vortex bursting at a point just before TE.

17. Jan 20, 2016

Ah, so the issue is the vortices bursting at different points then. Phenomena such as this, especially in low-Re (and likely laminar) situations, are highly-dependent on initial conditions. For example, minute differences in surface roughness or blemishes can play a major role in the development of the vortices. It seems rather likely then that the character of the surface on either side of your model is probably a bit different, so while the macroscopic shape of your model is symmetric, the surface characteristics are not, and that is causing your vortices to start with slightly different conditions and burst at different locations.

18. Jan 20, 2016

FactChecker

That is a statement about static stability that I question. There are certainly planes that do not have lateral stability if there is no tail, but that is much more complicated. I don't know about stable airflow in the wind tunnel. I am not a stability and controls expert, so I will withdraw.

19. Jan 20, 2016