# Hybrid spatial schemes for CFD: any downside to blending versus switching?

Aside from extra computational cost due to having to compute both fluxes over a certain region, is there any downside to blend two flux evaluations for a hybrid scheme in a finite volume method? The flux evaluation would look like this:

$\mathbf{F}_{i+\frac12} = \Lambda_{i+\frac12} \mathbf{F}^c_{i+\frac12} + (1 - \Lambda_{i+\frac12}) \mathbf{F}^u_{i+\frac12}$

The switch is based on a pressure and/or density gradient sensor depending on your application. $\mathbf{F}^c$ is a central scheme (McCormack, compact, ...) and $\mathbf{F}^u$ is an upwind scheme like a flux-difference splitting with a MUSCL reconstruction. Are there any issues in terms of numerics, conservative properties if I am blending the two schemes using a continuous function for $\Lambda$ as opposed to simply switching between schemes with $\Lambda$ valued as either 0 or 1?

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The classic example of switches-gone-wrong is the 1970 Murman-Cole scheme for transonic potential equations (fixed in 1973). If your switch, or blending, doesn't yield a telescoping sum then you violate conservation. It's straight forward to test. – tpg2114 Nov 27 '12 at 19:15
How will you be solving the equations? Implicit (pseudo-)time discretization? Switching could then have a negative effect on the iterative convergence. – chris Nov 29 '12 at 20:06
That would be using an explicit time scheme, either predictor-corrector or some basic Runge-Kutta. – FrenchKheldar Nov 29 '12 at 20:08