“…At the highest levels of FST, the bubble could be suppressed due to the boundary layer transitioning before the strong adverse pressure-gradient region, which promotes separation. However, the results could also suggest the suppression of laminar separation due to streaks (at least for the current experimental configuration), which is supported by the recent numerical investigation by Xu & Wu (2021). They found that free-stream vortical disturbances of moderate level prevent the separation in a boundary-layer flow over a plate or concave wall, suggesting that the strong mean-flow distortion associated with the nonlinear streaks or Görtler vortices prevents separation.…”
Section: Resultssupporting
confidence: 86%
“…A similar phenomenon could exist in our results, where critically energetic streaks generated by sufficiently elevated FST can suppress separation. In these cases, as in Xu & Wu (2021), we also observe streak amplitudes in the range of 10 %–15 % of the free-stream velocity, although, at lower amplitudes, an LSB is still present. Figure 20.Chordwise evolution of the disturbance profiles scaled with for chordwise positions of ( a ) 0.250, ( b ) 0.300, ( c ) 0.325, ( d ) 0.350, ( e ) 0.375, ( f ) 0.400, ( g ) 0.425, ( h ) 0.450, ( i ) 0.475, ( j ) 0.500.…”
Section: Resultssupporting
confidence: 85%
“…A similar phenomenon could exist in our results, where critically energetic streaks generated by sufficiently elevated FST can suppress separation. In these cases, as in Xu & Wu (2021), we also observe streak amplitudes in the range of 10 %-15 % of the free-stream velocity, although, at lower amplitudes, an LSB is still present.…”
Section: Co-existence Of a Modal And A Non-modal Instabilitiessupporting
confidence: 65%
“…They found that free-stream vortical disturbances of moderate level prevent the separation in a boundary-layer flow over a plate or concave wall, suggesting that the strong mean-flow distortion associated with the nonlinear streaks or Görtler vortices prevents separation. The simulations by Xu & Wu (2021) found that streaks with maximum amplitudes of approximately 12 % suppressed separation, whereas, at lower amplitudes, separation would still occur. A similar phenomenon could exist in our results, where critically energetic streaks generated by sufficiently elevated FST can suppress separation.…”
Experiments were conducted to study the transition and flow development in a laminar separation bubble (LSB) formed on an aerofoil. The effects of a wide range of free-stream turbulence intensity (
$0.15\,\%< Tu<6.26\,\%$
) and streamwise integral length scale (
$4.6\ {\rm mm}<\varLambda _{u}<17.2\ {\rm mm}$
) are considered. The co-existence of modal instability due to the LSB and non-modal instability caused by streaks generated by free-stream turbulence is observed. The flow field is measured using hot-wire anemometry, which showed that the presence of streaks in the boundary layer modifies the mean-flow topology of the bubble. These changes in the mean flow field result in the modification of the convective disturbance growth, where an increase in turbulence intensity is found to dampen the growth of the modal instability. For a relatively fixed level of
$Tu$
, the variation of
$\varLambda _{u}$
has modest effects. However, a slight advancement of the nonlinear growth of disturbances and eventual breakdown with the decrease in
$\varLambda _{u}$
is observed. The data show that the streamwise growth of the disturbance energy is exponential for the lowest levels of free-stream turbulence and gradually becomes algebraic as the level of free-stream turbulence increases. Once a critical turbulence intensity is reached, there is enough energy in the boundary layer to suppress the laminar separation bubble, resulting in the non-modal instability taking over the transition process. Linear stability analysis is conducted in the fore position of the LSB. It accurately models incipient disturbance growth, unstable frequencies and eigenfunctions for configurations subjected to turbulence intensity levels up to 3 %, showing that the mean-flow modification due to the non-modal instability dampens the modal instability.
“…At the highest levels of FST, the bubble could be suppressed due to the boundary layer transitioning before the strong adverse pressure-gradient region, which promotes separation. However, the results could also suggest the suppression of laminar separation due to streaks (at least for the current experimental configuration), which is supported by the recent numerical investigation by Xu & Wu (2021). They found that free-stream vortical disturbances of moderate level prevent the separation in a boundary-layer flow over a plate or concave wall, suggesting that the strong mean-flow distortion associated with the nonlinear streaks or Görtler vortices prevents separation.…”
Section: Resultssupporting
confidence: 86%
“…A similar phenomenon could exist in our results, where critically energetic streaks generated by sufficiently elevated FST can suppress separation. In these cases, as in Xu & Wu (2021), we also observe streak amplitudes in the range of 10 %–15 % of the free-stream velocity, although, at lower amplitudes, an LSB is still present. Figure 20.Chordwise evolution of the disturbance profiles scaled with for chordwise positions of ( a ) 0.250, ( b ) 0.300, ( c ) 0.325, ( d ) 0.350, ( e ) 0.375, ( f ) 0.400, ( g ) 0.425, ( h ) 0.450, ( i ) 0.475, ( j ) 0.500.…”
Section: Resultssupporting
confidence: 85%
“…A similar phenomenon could exist in our results, where critically energetic streaks generated by sufficiently elevated FST can suppress separation. In these cases, as in Xu & Wu (2021), we also observe streak amplitudes in the range of 10 %-15 % of the free-stream velocity, although, at lower amplitudes, an LSB is still present.…”
Section: Co-existence Of a Modal And A Non-modal Instabilitiessupporting
confidence: 65%
“…They found that free-stream vortical disturbances of moderate level prevent the separation in a boundary-layer flow over a plate or concave wall, suggesting that the strong mean-flow distortion associated with the nonlinear streaks or Görtler vortices prevents separation. The simulations by Xu & Wu (2021) found that streaks with maximum amplitudes of approximately 12 % suppressed separation, whereas, at lower amplitudes, separation would still occur. A similar phenomenon could exist in our results, where critically energetic streaks generated by sufficiently elevated FST can suppress separation.…”
Experiments were conducted to study the transition and flow development in a laminar separation bubble (LSB) formed on an aerofoil. The effects of a wide range of free-stream turbulence intensity (
$0.15\,\%< Tu<6.26\,\%$
) and streamwise integral length scale (
$4.6\ {\rm mm}<\varLambda _{u}<17.2\ {\rm mm}$
) are considered. The co-existence of modal instability due to the LSB and non-modal instability caused by streaks generated by free-stream turbulence is observed. The flow field is measured using hot-wire anemometry, which showed that the presence of streaks in the boundary layer modifies the mean-flow topology of the bubble. These changes in the mean flow field result in the modification of the convective disturbance growth, where an increase in turbulence intensity is found to dampen the growth of the modal instability. For a relatively fixed level of
$Tu$
, the variation of
$\varLambda _{u}$
has modest effects. However, a slight advancement of the nonlinear growth of disturbances and eventual breakdown with the decrease in
$\varLambda _{u}$
is observed. The data show that the streamwise growth of the disturbance energy is exponential for the lowest levels of free-stream turbulence and gradually becomes algebraic as the level of free-stream turbulence increases. Once a critical turbulence intensity is reached, there is enough energy in the boundary layer to suppress the laminar separation bubble, resulting in the non-modal instability taking over the transition process. Linear stability analysis is conducted in the fore position of the LSB. It accurately models incipient disturbance growth, unstable frequencies and eigenfunctions for configurations subjected to turbulence intensity levels up to 3 %, showing that the mean-flow modification due to the non-modal instability dampens the modal instability.
“…This could be due to the fact that increasing the level of FST with the roughness results in more energetic streaks (higher amplitudes), making the flow more resilient to separation. This was demonstrated numerically by Xu and Wu [56], who found that free-stream vortical disturbances of moderate level prevent separation in a boundary-layer flow over a plate or concave wall. It can be inferred that the strong mean-flow distortion associated with the streaks could prevent separation.…”
Section: The Combined Effects Of Roughness and Freestream Turbulencementioning
This experimental investigation studies the impact of streaks on two-dimensional laminar separation bubbles forming over an aerofoil. Streaks are introduced into the boundary layer using cylindrical roughness elements, and the resulting mean and unsteady flow fields are measured using hot-wire anemometry. The observed streaks generated by roughness exhibit analogous behavior to those generated by freestream turbulence, significantly altering the mean-flow characteristics of the bubble, including reductions in its length, height, and the introduction of spanwise velocity gradients. These mean flow modifications have a damping effect on convective disturbance growth. The experiments suggest the coexistence of modal instability due to the laminar separation bubble and non-modal instability due to streaks. We perform linear stability analysis at the fore position of the laminar separation bubble and find that by incorporating a spanwise wavenumber into the calculations, the experimental findings are reasonably modeled, indicating an oblique modal instability. To investigate the combined effect of roughness and the presence of freestream turbulence, we increase the turbulence level from the baseline in the presence of a roughness forcing configuration. We find that increasing the turbulence intensity leads to an enhancement of non-modal instability, accompanied by distinctive chordwise disturbance growth compared to lower freestream turbulence intensity levels. Moreover, increasing the turbulence intensity level in the presence of roughness not only bypasses the modal instability observed without the addition of turbulence but also suppresses laminar separation.
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