Sunroof Buffeting

At low speeds, flow resonance can occur when the shear layer coming off the front of a vehicle passes over its open sunroof and generates periodic pressure fluctuations within the cabin.

This phenomenon is known as sunroof buffeting, and can be modeled:

Recommended Process
Recommended Mesh Settings
Recommended Time-Step
Recommended Solver Settings

Recommended Process

The recommended process is:

Assess the resonance of the modeled cabin volume by performing a FRET (Frequency REsponse Test):
1. Apply a pressure disturbance to the sunroof aperture with no flow using an initial condition and field function.
2. Run the compressible solver with the same settings as a flow case to set up a compressible standing wave in the cavity.
3. Perform an FFT of the internal pressure-time signal, to identify the cavity resonance frequencies.
The resonance of the real vehicle cavity (from experiment) can be different from the FRET for various reasons, namely: panel impedance, leakages, or surface attenuation. Use an artificial compressibility factor, $ϕ$ , based on the ratio of experimental CFD frequencies to modify the response of the modeled cavity as follows:
$a^{'} = ϕ a_{0}$
(380)
${T^{'}}_{i n} = ϕ^{2} T_{i n}$
(381)
${p^{'}}_{r e f} = ϕ^{2} p_{r e f}$
(382)

where:

$a_{0}$ is the speed of sound in the fluid.
$T_{i n}$ is the inlet temperature.
$p_{r e f}$ is the reference pressure.

The speed of sound

a_{0}

is scaled so that the computed frequencies match the main frequencies from experimental setup. At the same time, hydrodynamic characteristics such as fluid density and velocities and Reynolds number remain the same.

Recommended Mesh Settings

Select cell size,

Δ_{s h e a r - l a y e r}

, based on criteria:


Criterion	Comments / Description	Formula	Typical Values
Shear layer	Full length of aperture and height of shear-layer oscillations.		$Δ_{s h e a r - l a y e r}$ = 4 mm

Recommended Time-Step

Select the time-step size

Δ t

, based on criteria:


Criterion	Comments / Description	Formula	Typical Values
Buffeting frequency $F_{B}$	Shear-layer oscillates over the opening. Buffeting frequency $F_{B}$ , is directly proportional to the free-stream speed, $U_{\infty}$ , and is in the range 20-30 Hz. See [76].	$F_{B} = \frac{U_{\infty}}{3 L} (N - \frac{1}{4})$ where: $N$ = oscillation mode $L$ = length of sunroof aperture	$Δ t_{F}$ = 1.0E-4 s.

Recommended Solver Settings

The recommended solver settings are:


	Velocity URF	Pressure URF	# Iterations Per Time-step
Default	0.8	0.2	5
Regular $Δ t$ = 1.0E-4 s. $Δ_{m i n}$ = 1–4 mm.	0.7	0.5	5–10
Aggressive $Δ t$ = 1.0E-4 s. $Δ_{m i n}$ = 1–4 mm.	0.9	0.7	5