Investigation and development of laminar-turbulent transition prediction and control methods

Responsible: I.V. Egorov

Participants: Fedorov A.V., Novikov A.V., Soudakov V.G., Ryzhov A.A., Obraz A.O., Pesetskaya E.A.

One of key problems of hypersonic aerodynamics is prediction of laminar-turbulent transition (LTT). Transition leads to increasing of the heat transfer by a factor of 3-8, which causes one either to increase the weight of thermal protection system (TPS) or design more complicated and expensive TPS. The aerodynamic stability, engine efficiency and flight range are also affected substantially. A large uncertainty in predictions of the transition onset line (more than 200%) makes it necessary to utilize conservative criteria that results in unreasonably large weight of TPS and low accuracy in computations of the vehicle aerodynamic characteristics.

Despite numerous studies of LTT in Russia, Europe and USA, this problem has not been solved yet. This is due to the followings challenges:

  1. Because LTT is sensitive to free-stream disturbances, the transition Reynolds numbers measured in wing tunnels differ from those in flight. Experimental database is limited and does not cover practical range of Mach and Reynolds numbers. It is difficult to extrapolate these data to the actual flight conditions.
  2. Transition depends on numerous factors, and many of them are not controlled or measured in experiments.
  3. The transition process can follow various scenarios, which depend on the body shape, surface roughness and the free-stream disturbance spectra.

This motivates theoretical and numerical studies of different unsteady processes associated with LTT. Such studies help to reveal the key mechanisms leading to transition, develop a physics based methods for prediction and control of the transition location.

Our research group investigates the LTT problem in the following directions:

1. Development of physics based methods for prediction of the LTT locus. A computational module is developed to predict the transition onset lines on 2D and 3D hypersonic configurations using the eN method. This module will allow us to calculate the growth rates and global amplification factors N for different unstable modes such as the Mack first and second modes and the cross-flow instability. It module will be equipped with a user-friendly GUI and will be compatible with Navier-Stokes solvers providing the mean flow solutions for 3D hypersonic configurations. Recently the first version of this module is tested using the transition data on cones at various angles of attack. It is planned to include new options for prediction of LTT induced by roughness and TPS imperfections. First, this will be done using available empirical criteria. Then, this empirical component will be reduced as theoretical and numerical models are developed to treat roughness-induced transition form first principles.

2. Numerical and theoretical studies of receptivity to external disturbances.This effort is focused on:

  1. Receptivity to thermal spots and free-stream turbulence
  2. Receptivity to free-stream noise associated with acoustic disturbances
  3. Receptivity to dust particles
  4. Receptivity to local forcing such as vibrations and blowing-suction through slots and holes

The results of this research form a foundation for further developments of more advanced LTT prediction method accounting for environmental disturbances - so-called amplitude method. This will allow us to reduce the empirical component of the transition prediction methodology and increase accuracy in computations of the transition location. In addition, the amplitude method will allow us to evaluate tolerable levels of TPS roughness as well as guide developments of laminar flow control techniques.

3. Numerical and theoretical studies of laminar flow control. Our group is working on laminar flow control methods aimed at delaying the transition onset. We showed theoretically and experimentally that thin microporous coatings and wavy surfaces lead to stabilization of the boundary-layer flow and, as a consequence, to significant increase of the transition Reynolds numbers. Recently we have been performing feasibility studies of laminar flow control using local heating or cooling of the hypersonic body surface. This research is conducted using stability analyses as well as direct numerical simulations.