I am simulating a transient, compressible blowdown scenario with no inlet flows and a single outlet, where the mass flow rate is fixed via a PCV and a choked orifice plate.

Has anyone incorporated a mass flow OUTLET boundary condition into Fluent 5? One approach would be to adjust the set pressure in a pressure outlet b.c. via a UDF to meet the specified mass flow rate. Any other suggestions?

Hi! Glenn,

This scheme file ought to work (it was written for UNS, so it might not work for Fluent 5, but it's worth a try). Let me know if you're not successful.

Graham

Put the text below this line in a file mfo.scm ...

;;; mfo.scm ;;; Mass-flow outlet boundary condition prototype.

;;; Copyright 1996 Fluent Inc. ;;; All Rights Reserved.

;;; Usage: ;;; ;;; - Set up your case and data as usual, using a pressure outlet ;;; for the outflow boundary type. Set the outflow pressure ;;; to a reasonable value and, for best results, partially converge ;;; the solution before proceding to the next step. ;;; ;;; - Load this file by typing (load 'mfo) in the console window. ;;; You will be prompted for a few configuration parameters. ;;; You can examine or change these parameters at any time with ;;; the /define/bc/mass-flow-outlet command. If you want, you ;;; can change the default configuration values below by editing ;;; this file. ;;; ;;; - Then just iterate as usual. The outlet pressure will be ;;; updated once every n iterations to achieve the target mass flow. ;;;

(define mfo/target-mf 10.0) ; target mass flow (define mfo/interval 5) ; update pressure every n'th iteration (define mfo/relax 1) ; relax pressure update (define mfo/min-pressure .1e5) ; limit pressure (define mfo/max-pressure 2.e5) (define mfo/verbose? #t) ; display status?

;;; end of user-configurable variables

(define mfo/name) (define mfo/area)

(define (mfo/ti-configure) (let ((po (get-threads-of-type 'pressure-outlet)))

(if (null? po)

(error "mass-flow-outlet: there are no pressure outlets defined"))

(set! mfo/name

(thread-name (car po)))

(format "Using ~a.\n" mfo/name))

(set! mfo/target-mf ;; (read-real "Target mass-flow (kg/s)" mfo/target-mf)

(read-with-units "Target mass-flow" mfo/target-mf 'mass-flow " ")) (set! mfo/interval

(read-real "Iteration update interval" mfo/interval)) (set! mfo/verbose?

(y-or-n? "Display update diagnostics?" mfo/verbose?)) (set! mfo/relax

(read-real "Pressure change relaxation factor" mfo/relax)) (set! mfo/min-pressure ;; (read-real "Minimum allowable gauge pressure (pascal)" mfo/min-pressure)

(read-with-units "Minimum allowable gauge pressure" mfo/min-pressure 'pressure " ")) (set! mfo/max-pressure ;; (read-real "Maximum allowable gauge pressure (pascal)" mfo/max-pressure)

(read-with-units "Maximum allowable gauge pressure" mfo/max-pressure 'pressure " "))

;; cache the surface area (set! mfo/area

(surface-area (cx-get-surface-ids (list mfo/name)))) )

;; add a /define/bc/mass-flow-outlet command

(ti-menu-insert-item! bc-menu (make-menu-item "mass-flow-outlet" #t

mfo/ti-configure

"Configure the mass flow outlet"))

;; build a new profile boundary condition with a constant value (define (constant-profile constant) (list (cons 'constant constant) '("" "")))

;; update the bc (define (mfo/update) (let* ((mfo (get-thread mfo/name))

(mf (- (cdr (assq 'mass-flow (thread-integrals (thread-id mfo))))))

(p (cdr (assq 'constant (send mfo get-var 'p))))

(rho)

(dp)

(new-p))

;; compute the average density on the thread

(client-set-node-values #f)

(client-fill-node-values 'density)

(set! rho (surface-average (cx-get-surface-ids (list mfo/name))))

;; adjust pressure using Bernoulli's

(set! dp (* mfo/relax

(/ mfo/target-mf rho mfo/area mfo/area)

(- mf mfo/target-mf)))

;; diagnostic message

(if mfo/verbose?

(format "mass-flow: actual = ~a, target = ~a, pressure change = ~a\n"

mf mfo/target-mf dp))

(set! new-p (min mfo/max-pressure

(max mfo/min-pressure

(+ p dp))))

;; update the boundary condition

(send mfo set-var! 'p (constant-profile new-p))

;; download to the solver

(send mfo update-solver-vars!)

))

;; automatically update the bc each iteration

(define (mass-flow-outlet-monitor niter . args) (if niter

(if (zero? (remainder niter mfo/interval))

(mfo/update))) '())

(cancel-solution-monitor 'mass-flow-outlet-monitor) (register-solution-monitor 'mass-flow-outlet-monitor mass-flow-outlet-monitor)

;; check the configuration (mfo/ti-configure)

;;; end-of-file

Thanks alot Graham!

Is there some way to disable the formatting of posts so that code can be posted legibly?

I have tried various html tags (pre, code, plaintext) but the indentation and line breaks are still being changed.

Thanks, Chris

No, there is no way to do that currently, sorry. All posts are parsed in order to optimize the apperance of plain-text on different browsers. The parsing removes all line-breaks so that the browser wraps lines automatically based on window-width. Double line-breaks are translated into paragraph-breaks. This works okay on plain-text, which is the vast majority of all posts, but when people try to post tables, source-code, indendation etc. things will break. I know that it is annoying. I will try to find a solution.